It has long been known to measure elastic strain resulting from load-induced stresses of elements, such as beams, columns, diaphragms, and the like, through the use of electrical strain gages for the accurate translation of the exhibited strains into convenient related variations in electrical impedances. Wire, foil, and semi-conductor type gages find widespread application in such transducer devices and lend themselves well to manufacture in highly miniaturized form, suitable for installation upon small surface areas of such transducer devices.
Commonly, the transducers are designed and fabricated as self-contained devices intended to satisfy the needs of a variety of installations, but, in general, they tend to be accurately responsive only when the applied loading is carefully centered with respect to a predetermined axis. In small size transducer units, auxiliary diaphragms are often employed to suppress undesirable lateral deflections. In larger transducer assemblies, such as those embodied in massive weighing platforms or the like, potentially troublesome side forces may be resisted by strong, but somewhat elastic, structural members, which are sometimes referred to as flexures or stay rods. In these arrangements, flexible guide means are provided, which effectively parallel the load cells or transducers. In load cells of this particular design, the critical spring characteristics of the sensor are modified by those of the parallel flexure members or stay rods, and measurement accuracy and linearity can be seriously impaired by any erratic or non-linear behavior of these members. More recently, some load cells have used force-sensitive resonators to measure the frequency output as the function of the applied load.
However, these prior art load cells, as described hereinabove, are limited in resolution by anelastic creep and static strain hysteresis, which is apparent when low shear stresses, in relation to bending stresses, exist within flexing elements of the load cells. Further, in these prior art load cells, side forces and effects, other than the forces to be measured, must be offset by bridge-circuit balancing. Bridge-circuit balancing requires that the placement of the strain gages be exact and also requires that the gage factors of all of the gages be the same. It has been the general custom and practice to offset these inaccuracies by custom machining these prior art load cells.